This project is concerned with applying statistical mechanics to an understanding of the phase transitions in lipid bilayer membranes. First, we are applying a theory of the chain melting transition in lipid bilayers to the interpretation of experimental transition temperatures for various phospholipids and of phase diagrams for various two component bilayers. The object is to determine the molecular basis of the observed differences between phosphatidyl cholines and phosphatidyl ethanolamines and between lipids with saturated hydrocarbon chains, trans-unsaturated chains, and cis-unsaturated chains. To obtain additional data for applying the theory, we are also measuring transition temperatures and determining phase diagrams using spin and fluorescence probes. Secondly, we are developing a theory of ion binding to membrane surfaces. The theory takes into account the competitive binding of protons and of divalent cations and the influence of the electrical double layer upon the binding. This theory will be applied to interpreting measured electrical properties of bilayers and monolayers and the effects of pH and divalent ion concentrations on the chain melting transition of bilayers.